1. Field of the Invention
This invention relates to a developer carrying member used to develop electrostatic latent images formed on an electrostatic latent image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric in electrophotography, and a developing assembly in which such a developer carrying member is set.
2. Description of the Related Art
A number of methods are conventionally known as methods for electrophotography. In general, copies are obtained by forming an electrostatic latent image on an electrostatic latent image bearing member such as a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a toner to form a toner image, transferring the toner image to a transfer material such as paper as occasion calls, and thereafter fixing the toner image onto the transfer material by the action of heat and/or pressure.
In recent years, electrophotographic apparatus such as printers and copying machines have been made high-resolution as a trend of techniques. For example, those which hitherto have a resolution of 300 dpi or 600 dpi are being replaced by those having a resolution of 1,200 dpi or 2,400 dpi. Accordingly, developing systems are correspondingly required to achieve a higher minuteness.
In addition, in the field of electrophotography, color image formation is on rapid progress. Color images are formed by development performed by superimposing yellow, magenta, cyan and black four-color toners appropriately, and hence the respective-color toners are sought to have a higher developing performance than those in monochrome image formation. Accordingly, it has become important to control the chargeability of color toners uniformly.
It, however, is difficult to regulate the charging of toners, and various measures are being taken on toners. Nevertheless, especially in the case of one-component developing systems, it is demanded to further improve toners in their charge uniformity and running stability of charging.
In particular, as the developer carrying member such as a developing sleeve is repeatedly rotated, what is called charge-up tends to occur, which is a phenomenon in which the charge quantity of the toner coated on the developing sleeve becomes too high because of contact with the developing sleeve. Once it occurs, the toner is very strongly attracted to the developing sleeve surface to come to stand immobile on the developing sleeve surface to become not movable to the latent images on the photosensitive drum from the developing sleeve. If such a phenomenon of charge-up occurs, the toner can not easily be charged at its upper-layer portion in a toner layer on the developing sleeve surface. For this reason, the toner that participates in development decreases, and hence this tends to cause thin line images and a decrease in image density of solid images. A phenomenon may further come about in which such toner not properly charged because of the phenomenon of charge-up flows out onto the developing sleeve to cause line-shaped, blotchy or wavy non-uniformity.
Toners are recently made to have small particle diameter in order to make electrophotographic apparatus digital and make them achieve much higher image quality, and made to have closely spherical particle shape in order to reduce waste toners.
However, where a toner having been made to have a small particle diameter and made spherical is used, the toner is improved in fluidity, but on the other hand shows a tendency that it is difficult for the toner to be coated on the developing sleeve in a uniform thin layer. Accordingly, in order for such a toner to be uniformly coated on the developing sleeve, it is necessary to enhance toner control power by using a developer layer thickness control member such as a developing blade. Especially in a low-temperature and low-humidity environment, the charge quantity of toner per unit mass tends to increase. Hence, the toner may non-uniformly be charged to tend to cause image density decrease and image density non-uniformity, and also the toner having strongly adhered onto the developing sleeve may grow to melt-adhere thereto to cause faulty images such as fog and vertical streaks.
In a high-temperature and high-humidity environment, because of the use of materials which make the toner readily flowable by physical force and heat in order to improve low-temperature fixing performance, melt adhesion of the toner to the developing sleeve surface and developing blade surface is accelerated as a result of the fact that the toner is repeatedly rubbed between the developing sleeve and the developing blade with repetition of copying and then undergoes a stress. This tends to seriously cause faulty images such as fog, image density non-uniformity and vertical streaks.
Meanwhile, as a method for resolving such a phenomenon, Japanese Patent Application Laid-open No. H08-240981 discloses use of a developing sleeve comprising a metallic substrate and provided thereon with a resin coat layer in which a conductive material or solid lubricant such as carbon black or graphite particles is dispersed.
The use of such a developer carrying member is seen to make the above phenomenon of charge-up vastly less occur, and further to make the toner less melt-adhere to the developing sleeve surface in a high-temperature and high-humidity environment.
However, in the above developer carrying member, the profile of surface unevenness of the resin coat layer is not uniform and also the lubricity of the resin coat layer surface is not sufficiently uniform, and hence it is necessary to more improve charge characteristics by which the toner can uniformly and quickly be charged. Also, in long-term and long-running service, especially where the toner having been made spherical is used, part of the resin coat layer surface stands easily causative of toner melt adhesion because of an influence of non-uniform hills in unevenness of the resin coat layer surface and because of such non-uniform lubricity of the resin coat layer surface. Accordingly, since the toner may come to be non-uniformly triboelectrically charged to cause fog seriously and cause image density non-uniformity, sleeve ghosts and line-shaped image defects, there is room for improvement.
In particular, in the case when the developer carrying member having the resin coat layer in which the solid lubricant such as graphite particles has been dispersed is used, the resin coat layer surface has lubricity due to the scaly structure of graphite, and hence brings out a sufficient effect against sleeve ghosts which may be caused by the charge-up. However, the graphite particles are not easily uniformly be dispersed in the coating resin because of their shape which is scaly and amorphous and the particles have particle diameters as large as 2 to 30 μm and also have a broad particle size distribution. As the result, a resin coat layer which contains only the graphite particles in a large quantity may have a non-uniform surface profile to tend to cause wear and liberation of the graphite particles themselves and any local poor lubricity, at the resin coat layer surface. Hence, where the developer carrying member is used in long running, its resin coat layer may come to wear to change in surface roughness and surface composition and also cause toner contamination, to tend to cause faulty transport of toner and make the toner charged non-uniformly.
A method is also disclosed in which, as a conductive material, specific carbon black having a high electrical conductivity, such as KETJEN BLACK or acetylene black, is used in the resin coat layer of the developer carrying member. For example, Japanese Patent Application Laid-open No. 2001-331032 discloses a developing roller having a resin coat layer which contains KETJEN BLACK. Japanese Patent No. 2795168 also discloses a developing roller having an elastic layer which contains acetylene black.
Carbon black commonly has a small particle diameter and a large specific surface area and its structures stand developed. Hence, it has a high DBP oil absorption. According to the internet website of Lion Corporation, “Introduction of carbon black KETJEN BLACK”, searched on Sep. 21, 2002, on Internet <URL:http://www./lion.co.jp/chem/jn/sectop/carbon/k intro.htm>, KETJEN BLACK EC300J has a DBP oil absorption of as high as 360 ml/100 g; and KETJEN BLACK EC600JD, 495 ml/100 g. However, the carbon black having a high DBP oil absorption like KETJEN BLACK can not easily uniformly be dispersed in the resin coat layer. Hence, the effect of keeping the toner from charging up is not sufficiently brought out to tend to make the toner charged non-uniformly, and also tend to cause a leak of development bias, starting from the agglomeration of KETJEN BLACK. The carbon black like KETJEN BLACK may also insufficiently provide the resin coat layer with lubricity to tend to make the toner melt-adhere to the resin coat layer surface during long-term service. In this aspect as well, the toner tends to come charged insufficiently. Further, KETJEN BLACK tends to vary in resistivity depending on humidity, and hence may come to have a low charge-providing performance to toner especially in a high-temperature and high-humidity environment.
Meanwhile, the acetylene black is carbon black obtained by partial combustion of acetylene, and is usually produced by thermal decomposition or by a combustion process. As described in “Properties and Optimum Formulation of Carbon Black and Its Application Techniques”, page 284, published by K.K. Gijutu Joho Kyokai on May 26, 1997, this acetylene black as well has a DBP oil absorption as high as 250 ml/100 g, and also its structures stand developed. Such structures come into contact with one another to form conducting paths to thereby manifest the electrical conductivity. However, because of the one having a high DBP oil absorption, like the above KETJEN BLACK, it can not easily be dispersed in the resin coat layer to tend to make it insufficient to keep the phenomenon of charge-up of toner from occurring, and tend to make the toner charged non-uniformly. In addition, the resin coat layer is insufficiently provided with the lubricity, and hence the toner tends to melt-adhere to the resin coat layer surface during long-term service, also tending to make the toner charged non-uniformly.
Japanese Patent Application Laid-open No. H07-013415 discloses a developing roller comprising a substrate and provided thereon a resin coat layer in which an ionic conductor has been dispersed in addition to a conductive fine powder such as carbon black and crystalline graphite particles. In this method, the addition of the ionic conductor is seen to be effective in keeping the phenomenon of charge-up of toner from occurring. However, the ionic conductor has a high humidity dependence. Hence, it follows that the resin coat layer varies greatly in its resistivity to cause a decrease in charge quantity of toner especially in a high-temperature and high-humidity environment. Thus, this method is still insufficient in regard to image density stability and fog during long-term service.